Compact heat exchange systems using, for example, a burner positioned inside a plenum formed by a heat exchanger which has a surface area contacting the products of combustion which is substantially larger than the surface area contacting a fluid to be heated by the products of combustion can economically extract heat from the products of combustion so efficiently that condensation from flue gas products occurs in the heat exchanger and causes deposits to be formed either by interaction with the heat exchanger surface coating or by deposition of particulate matter from the products of combustion. Such deposits can result in partial plugging of the passages of the heat exchanger through which the products of combustion pass which, in turn, further reduces the heat supplied to the heat exchanger in these regions thereby increasing the amount of condensation.
Formation of such deposits may be particularly severe when excess air is supplied to the burner to reduce the emission of pollutants from the products of combustion since this reduces the temperature of the products of combustion passing through the heat exchange passages.
In addition, the temperature at the surface of some parts of the burner heat exchanger may produce condensation deposits if the fluid being heated enters the heat exchanger at too low a temperature and at too high a rate thereby overcooling the burner heat exchanger.
In addition, a heating system may be combined with a cooling system in a package unit, for example, for external mounting in the back yard or on the roof of a home, and full advantage may then be taken of a common blower for blowing air through the home from the package unit, with said air being either heated by an air heater or cooled by an evaporating heat exchanger. Previous to this invention, hot air heaters have been used in such package units so that the cooled air was blown through the hot air heat exchanger during periods when no heat is being supplied to the heater and, as a result, air at or near the dew point of the air, or contaminants thereof, formed deposits by reaction or otherwise on the surface of the hot air heater which upon being heated caused accelerated corrosion thereof. Therefore, the life of such a hot air heater was reduced, particularly if the hot air heater was designed to operate near the upper limit of its safe operating range to achieve a sufficiently compact size to fit in an economically feasible heating and cooling unit.
In addition, while materials and coatings for the cooling heat exchanger may be properly chosen and designed for long life since this heat exchanger may be placed on the input side of the blower and, hence, never subject to overheating, a hot air heater cannot normally be economically coated with a material which will protect against dew point corrosion and will also stand elevated temperatures without substantial expense and difficulty.
Furthermore, derating such a hot air heater to a point where adequate life is obtained makes such units bulky and heavy and renders such units overly expensive.